JPH06166580A - Ceramic foam and its production - Google Patents

Abstract

PURPOSE:To simply produce a ceramic foam having heat resistance and strength. CONSTITUTION:A silicon-containing thermoplastic ceramic precursor (e.g. polycarbosilane or polysilastyrene) is mixed with a perhydropolysilazane (preferably having <=1500 number-average molecular weight) in a ratio of number of silicon atoms of (1/19) to (1/19) and the mixture is thermally decomposed at 300-1700 deg.C.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a ceramic foam and a method for producing the same. This ceramic foam can be preferably applied to the fields of heat insulation industry, vehicles such as catalyst carriers for automobiles, and fields requiring heat insulation and heat resistance at high temperatures.

[0002]

2. Description of the Related Art Plastic foams such as expanded polystyrene and expanded polyurethane have excellent heat insulating properties, shock absorbing properties, electrical insulating properties, and the like, and have low thermal conductivity and moisture permeability. Because of its light weight and other characteristics, it is widely used as a heat insulating material for roofs and cooling / heating equipment, a heat insulating material for freeze prevention, electric products, sports equipment and the like.

As a heat insulating material for high temperatures, a molded body of a needle-shaped crystal substance such as asbestos has been used. Various attempts have been made to obtain a foam structure of ceramics.

[0004]

Plastic foams cannot be used at high temperatures, and needle-like crystal substances such as asbestos are harmful to the human body. Further, the conventional method for obtaining a ceramic foam structure is complicated, and there is a problem that a uniform thin structure such as a plastic foam cannot be obtained.

An object of the present invention is to solve such problems and provide a uniform ceramic foam having excellent heat resistance and strength and a simple method for producing the same.

[0006]

To achieve the above object, the present invention provides a silicon-containing thermoplastic ceramic precursor and perhydropolysilazane in a silicon atom number ratio of 19/1 to 1.
A method for producing a ceramic foam, which comprises mixing the mixture in a ratio of / 19 and firing to obtain a ceramic foam.
And silicon, nitrogen and carbon as essential components, oxygen and hydrogen as optional components, and the ratio of each element is represented by atomic ratio, N
/Si=0.01 to 1, C / Si = 0.1 to 1.5, O
/Si=0.3 or less and H / Si = 0.1 or less,
Amorphous consisting essentially of silicon, nitrogen and carbon or amorphous consisting of silicon, nitrogen and carbon with a crystal grain size of 500Å
Provided is a ceramic foam comprising the following aggregate of β-SiC crystalline fine particles or a mixed system.

In the process of diligent efforts to achieve the above object, the inventors of the present invention mixed the thermoplastic ceramic precursor polymer with the thermosetting perhydropolysilazane having a relatively low molecular weight and then heated the mixture to give an amorphous material. Or crystalline Si,
It has been found that a homogeneous ceramic foam containing C and N as the main components can be obtained. Such ceramic foams are obtained because the softening of the thermoplastic ceramic precursor polymers, the hardening of the perhydropolysilazane and the thermal decomposition of these polymers occur at about the same time.

The perhydropolysilazane used in the present invention is a polysilazane having a skeleton composed of a repeating unit represented by the formula-(SiH ₂ NH)-, that is, a side chain of which is all hydrogen atoms, and is mainly chain-like. , A cyclic portion, and may have a crosslinked structure. As such a perhydropolysilazane, a perhydrosilazane oligomer obtained by ammonolysis of a dichlorosilane / pyridine complex (Japanese Patent Publication No. 63-1632).
5), an inorganic high polymer obtained by heating this oligomer in a basic solution (JP-A-1-138108), and a modified polysilazane obtained by reacting the oligomer with ammonia (JP-A-1-138108). No. 138107) or the like can be used. Perhydropolysilazane is S
It has i-H and N-H bonds, is highly reactive, and is thermosetting, and is optimal for the purpose of the present invention.

The molecular weight of perhydropolysilazane is preferably such that it forms a liquid state, and the number average molecular weight Mn is several 1500 or less, more preferably 1200 or less. In the present invention, since the thermal decomposition gas of perhydropolysilazane is used to cause foaming at the time of heating, if the molecular weight of perhydropolysilazane is too large, it does not volatilize even when thermally decomposed, and it is difficult to generate a foaming gas, which is preferable. Absent. The molecular weight (number average) Mn is 360 or more, preferably 5
00 or more is preferable. If the molecular weight is too low, there are many volatile components,
The thermal decomposition yield becomes low and a good foam cannot be obtained. On the other hand, if the molecular weight is too low, the viscosity is low, and after mixing with the thermoplastic polymer, the perhydropolysilazane and the thermoplastic polymer are easily decomposed, so that a good foam cannot be obtained.

The thermoplastic silicon-containing ceramic precursor used in the present invention is one which is softened by heat and foams by a pyrolysis gas, and a foam cannot be obtained without the presence of a thermoplastic polymer. It may be polysilane containing silicon, polycarbosilane, polysiloxane, polysilazane, and the like, and in particular, silicon carbide-based precursor polymers such as polycarbosilane, polysilastyrene, polycarbosilastyrene, methylpolysilane, phenylpolysilane, and polysilane. Titanocarbosilane, polyzirconocarbosilane, polydisililazane and the like can be preferably used.

The molecular weight of the thermoplastic silicon-containing ceramic precursor may be liquid or solid, and the number average molecular weight is 60.
Those in the range of 0 to 2000 are preferable. If the molecular weight is too small, the thermal decomposition yield is low, and a good foam cannot be obtained.
If it is too large, it will gel and be difficult to handle. In the present invention, the perhydropolysilazane and the silicon-containing thermoplastic ceramic precursor are mixed in a ratio of the number of silicon atoms of 19/1 to 1/19. When the amount of the thermoplastic ceramic precursor is too large, the thermosetting property is poor, so the bubble diameter becomes too large, and when the amount of the perhydropolysilazane is too large, the thermoplasticity is insufficient, so the bubble size is uneven. Because. A more preferable ratio is 8/1 to 1/8.

Although the mixing of the perhydropolysilazane and the thermoplastic ceramic precursor can be carried out without solvent,
If each polymer is dissolved in a non-reactive solvent and then mixed and the solvent is distilled off, more uniform mixing can be achieved. As a non-reactive solvent,
Halogenated hydrocarbons such as methylene chloride, chloroform, carbon tetrachloride, bromoform, ethylene chloride, ethylidene chloride, trichloroethane, tetrachloroethane, etc.,
Ethyl ether, isopropyl ether, ethyl butyl ether, butyl ether, 1,2-dioxyethane,
Ethers such as dioxane, dimethyldioxane, tetrahydrofuran, tetrahydropyran, pentane, hexane, isohexane, methylpentane, heptane, isoheptane, octane, isooctane, cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, benzene, toluene, xylene, ethylbenzene. And other hydrocarbons.

After mixing the perhydropolysilazane and the thermoplastic ceramic precursor, they are fired (heat-treated) and thermally decomposed. The thermal decomposition temperature may be in the range of 300 ° C to 1700 ° C. If it is carried out at a temperature higher than 1700 ° C., silicon nitride derived from perhydropolysilazane will volatilize and exhibit high crystallinity, so that a foam having excellent strength cannot be obtained. Further, at 300 ° C. or lower, strength is not obtained because of insufficient ceramicization, and a small amount of gas is generated due to thermal decomposition, so that a good foam cannot be obtained. Preferably 6
It is within the range of 00 ° C to 1500 ° C.

The heating rate may be in the range of 1 ° C./minute to 50 ° C./minute. A good ceramic foam can be obtained because softening of the thermoplastic polymer, melting of the perhydropolysilazane, and foaming of the polymer by pyrolysis gas occur at approximately the same temperature, but during pyrolysis, a certain time residence in this temperature range is necessary. At a heating rate of more than 60 ° C./minute, uniform foaming cannot be obtained, and at a heating rate of less than 1 ° C./minute, foaming is insufficient and a foam cannot be obtained. Further, if the heating rate is less than 1 ° C./minute, the productivity is poor. A more preferable temperature rising rate is in the range of 2 ° C / minute to 20 ° C / minute.

The firing atmosphere is not particularly limited, but N ₂ ,
Ar, He or the like may be used. Further, if it is pyrolyzed in O ₂ or air, a ceramic foam containing O as a component can be obtained. It should be noted that when the amount of pyrolysis gas is insufficient, addition of a foaming aid facilitates formation of a foam. Examples of the foaming aid include organic polymers such as polystyrene, polyethylene, polypropylene, polyurethane, ABS resin, vinyl chloride, and cellulose acetate, and one or more of these can be used.

The cells of the ceramic foam thus obtained are preferably in the range of 0.1 to 2 mm in diameter, and in this range, the foam has strength. If the uniformity of bubbles is in the range of 1 ° C./min to 50 ° C./min depending on the heating rate, a uniform distribution of bubbles can be obtained. The size of the cells can be adjusted by the mixing ratio of perhydropolysilazane and thermoplastic polymer. That is, as the amount of perhydropolysilazane increases, the bubbles in the foam become smaller, and as the amount of thermoplastic polymer increases, the bubbles become larger.

Further, according to the present invention, there is provided a silicon carbide based ceramic foam as the ceramic foam manufactured by the above manufacturing method. This ceramic foam is an inorganic molded body containing silicon, nitrogen and carbon as essential components and oxygen and hydrogen as optional components, and its crystallinity may be crystalline or amorphous. Those that are amorphous are preferred. That is, the amorphous or crystallite size (measured using the X-ray diffraction half width method (JONES method)) by X-ray diffraction analysis is 50 in all directions.
Those containing a fine crystal phase (β-SiC) of 0 Å or less are preferable.

The ratio of each element constituting the silicon carbide molding of the present invention is expressed by atomic ratio: N / Si 0.01 to 1 C / Si 0.1 to 1.5 O / Si 0.3 or less And the preferable atomic ratio is N / Si 0.05 to 0.8 C / Si 0.2 to 1.2 O / Si 0.25 or less. A more preferable atomic ratio is N / Si 0.1 to 0.7 C / Si 0.5 to 1.0 O / Si 0.2 or less.

When the element ratio is not included in the above range,
Mechanical strength, heat resistance and oxidation resistance decrease.

[0020]

By softening a mixture of a silicon-containing thermoplastic ceramic precursor and a thermosetting perhydropolysilazane in a micro state, softening of the silicon-containing thermoplastic ceramic precursor, hardening of the perhydropolysilazane, and thermal decomposition A good ceramic foam can be obtained by the gas generation occurring almost at the same time.

[0021]

The ceramic foam of the present invention has a structure in which bubbles are uniformly dispersed, and also has excellent heat resistance. According to the method for producing a ceramic foam of the present invention, the heat insulating material can be easily produced in a desired shape without using a substance harmful to the human body such as asbestos. Further, the foam having open cells can be used as a catalyst carrier which requires heat resistance. There is an advantage that these high temperature heat insulating materials and catalyst carriers can be manufactured safely and easily.

[0022]

EXAMPLES Reference Example 1 A four-necked flask having an internal volume of 11 was equipped with a gas blowing tube, a mechanical stirrer, and a dewar condenser. After replacing the inside of the reactor with deoxygenated dry nitrogen, 490 ml of degassed dry pyridine was put into a four-necked flask, and this was ice-cooled. Next, 51.6 g of dichlorosilane was added to produce a white solid adduct (SiH ₂ Cl ₂ · 2C ₅ H ₅ N).
Was generated. The reaction mixture was ice-cooled and, while stirring, 51.0 g of purified ammonia was bubbled through a sodium hydroxide tube and an activated carbon tube.

After completion of the reaction, the reaction mixture was centrifuged, washed with dry pyridine and then filtered under a nitrogen atmosphere to obtain 850 ml of a filtrate. The solvent was distilled off from 5 ml of the filtrate under reduced pressure to give a resin solid of perhydropolysilazane 0.102.
g was obtained. The number average molecular weight of the obtained polymer is GP
It was 980 when measured by C. In addition, the IR (infrared absorption) spectrum of this polymer (solvent: dry o
-Xylene; Perhydropolysilazane concentration: 10.2
g / 1), the wave number (cm ^-1 ) 3350 and 11
It was confirmed to exhibit NH-based absorption of 75; SiH-based absorption of 2170; SiH and SiNSi-based absorption of 1024 to 820. Also ¹ H of this polymer
NMR (proton nuclear magnetic resonance) spectrum (60 MHz,
When the solvent CDCl ₃ / reference substance TMS) was examined, it was confirmed that all exhibited a wide absorption. That is δ
4.8 and 4.4 (br, SiH); 1.5 (br, NH)
Was confirmed to be absorbed.

Example 1 10 g of polycarbosilane (manufactured by Shin-Etsu Chemical Co., Ltd.) was added to 100 ml of o.
-Dissolved in xylene, 200 ml of an o-xylene solution of perhydropolysilazane obtained in Reference Example 1 (12 wt%, perhydropolysilazane 21 g) was added and mixed. When the solvent was removed by a rotary evaporator, a pale yellow solid was obtained. Thermal decomposition of this solid in Ar at 5 ° C / mi
The temperature was raised to 1200 ° C. by n and the operation was performed at 1200 ° C. for 3 hours.

A black ceramic foam was obtained with a yield of 80% by weight. A micrograph (× 20) of the obtained ceramic foam is shown in FIG. The foam was amorphous by powder X-ray diffraction. The result of elemental analysis is Si:
It was 62.9% by weight, C: 13.5% by weight, N: 23.6% by weight.

The bubbles of this foam are 0.2 to 0.5.
With uniform dimensions in the mm diameter range, density 0.49 g / c
It was m ³ , and the porosity was 80%. Example 2 40 g of polysilastyrene (manufactured by Nippon Soda) was added to 300 ml of o.
-Dissolved in xylene, 30 ml of an o-xylene solution of perhydropolysilazane obtained in Reference Example 1 (12 wt%, perhydropolysilazane 3.2 g) was added and mixed. When the solvent was removed by a rotary evaporator, a pale yellow solid was obtained. Pyrolysis of this solid in N ₂ at 10 ° C /
The temperature was raised to 1500 ° C. at min, and the temperature was maintained at 1500 ° C. for 1 hour.

A black ceramic foam was obtained with a yield of 75% by weight. Broad X-ray analysis by powder X-ray diffraction
SiC (crystallite diameter 40Å) was observed. The elemental analysis results are Si: 55.3% by weight, C: 40.6% by weight, N:
It was 4.1% by weight. Foam bubbles are 0.6-1.0
Uniform in mm diameter range, porosity 82%, density 0.43 g /
It was cm ³ .

The pale yellow solid obtained by mixing a polycarbosilane with perhydropolysilazane in the same manner and proportions as Comparative Example Example 1 A
In r, the temperature was raised to 1200 ° C at 0.1 ° C / min, and
Hold at 0 ° C. for 3 hours. Black ceramics were obtained with a yield of 82% by weight. No uniform foaming was observed, the density was 1.90 g / cm ³ and the porosity was 79%.

[Brief description of drawings]

FIG. 1 is a photomicrograph showing the structure of a ceramic foam produced in an example.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] July 28, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is a foam ceramic material produced in an example.
2 is a micrograph showing the structure of

Claims (2)

[Claims] 1. A silicon-containing thermoplastic ceramic precursor and perhydropolysilazane in a silicon atom number ratio of 19/1 to 1
A method for producing a ceramic foam, which comprises mixing and firing at a ratio of / 19 to obtain a ceramic foam. 2. Silicon, nitrogen and carbon as essential components, oxygen and hydrogen as optional components, and the ratio of each element is expressed by atomic ratio, N / Si = 0.01 to 1, C / Si = 0. 1 to
1.5, O / Si = 0.3 or less, H / Si = 0.1 or less, and substantially amorphous including silicon, nitrogen and carbon or amorphous including silicon, nitrogen and carbon. A ceramic foam comprising an aggregate or mixed system of β-SiC crystalline fine particles having a crystal grain size of 500 Å or less.